package LuoGu;

import java.util.Scanner;

public class P5708 {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        double a=0,b=0,c=0;
        a=sc.nextDouble();
        b=sc.nextDouble();
        c=sc.nextDouble();
        double p=(double)(a+b+c)/2.0;
        double s=Math.sqrt(p*(p-a)*(p-b)*(p-c));
        System.out.printf("%.1f",s);
    }
}
